1. Field of the Invention
The present invention relates to an image sensor. More specifically, the present invention relates to a MOS type image sensor that is used as a contact type image sensor (CIS) for reading an image such as a document, and an image reading apparatus such as a facsimile machine utilizing the image sensor.
2. Description of the Related Art
Conventionally, two types of image sensors are used with an image reading apparatus such as a facsimile machine; a CCD type image sensor and a MOS type image sensor. Of those, a MOS type linear image sensor is often used as a contact type image sensor
FIG. 17 shows a conventional MOS type linear image sensor 100. This image sensor 100 includes a plurality of photoelectric conversion elements (phototransistors or photodiodes) 101 that convert light reflected by the surface of a document into an electric signal as an analog image signal, a plurality of switching elements (analog switches) 102 for reading out the analog image signal from the photoelectric conversion element 101, and a control unit (a shift register) 103 for controlling time-sequentially the switching elements 102. These elements 101, 102 and 103 are integrated into a single chip LSI.
However, the above-mentioned conventional image sensor, in which a lot of photoelectric conversion elements are arranged in a main scanning direction, has following problems to satisfy requirements for high resolution and for high speed reading.
Firstly, analog switches that are capable of high speed switching operation are required as the switching elements 102. In addition, an analog amplifier 104 that can amplify the read image signals from the photoelectric conversion elements at high speed is required.
Secondly, a shift register that includes as many flip-flop circuits as the switching element 102 is necessary as the control unit 103. Accordingly, a large area for arranging the flip-flop circuits is necessary, so that the chip size becomes large.
Thirdly, as a frequency of driving clock increases, it is difficult to eliminate high frequency components of the clock signal. As a result, a problem of electromagnetic interference (EMI) noise occurs.
Fourthly, the following case will be supposed in which an image sensor having a lot of photoelectric conversion elements arranged in the main scanning direction is required in order to read a high resolution image. If a reading condition is changed so as to satisfy low resolution reading requirement, it is required to thin-out the image signals or to average the levels of the image signals in order to convert the resolution after reading at high resolution. Therefore, in spite of low resolution reading, time period to read the image is long. Thus, it is difficult to support a request for placing a higher priority on reading speed than resolution.